114 Zoologica: N. Y. Zoological Society. [I; 3 



The exclusively arboreal type of bird is so characteristic of 

 the Orinocan region that it is worth while to examine it from the 

 standpoint of adaptive radiation. We may illustrate the arbo- 

 real specialization by a diagram, the condition of each division 

 of which offers a good field for future observation. (Fig. 37.) 



We find Opisthocomus — a wholly arboreal form — confined 

 usually to the lower branches. The Orders Galliformes, Grui- 

 formes, Anseriformes and Pelecaniformes, which, in more tem- 

 perate zones are chiefly terrestrial, are predominantly arboreal 

 in the Orinoco region. 



Ascending the trunks of the trees we find two dominant 

 groups, exhibiting parallel specialization — the woodpeckers and 

 the woodhewers. Confined to the higher branches and especial- 

 ly adapted to a scansorial life, we have the great groups of par- 

 rots; while hummingbirds and American trogons are adapted 

 for aerial feeding, obtaining insects in the one case, and fruit in 

 the other, from the branch tips. 



